Rechargeable electrochemical cells, also known as secondary cells, typically include an anode, a cathode and an electrolyte. In many commercially available secondary cells the anode includes an alkali metal; the electrolyte is a solution containing an electrolytic salt which is usually an alkali metal as an anode; and the cathode includes an electrochemically active material, such as compound of a transition metal. During use, electrons pass from the anode through exterior connecting circuitry to the cathode and alkali metal ions from the anode pass through the electrolyte to the cathode where the ions are taken up, with the release of electrical energy. During charging, the flow of electrons and ions are reversed.
In the design of secondary cells two issues are of importance. On the one hand the cell must be safe; on the other hand the cell must have good performance, meaning that it must be able to produce energy and be capable of being cycled (charged and discharged) numerous times.
To meet the energy requirement of the secondary cells, the use of lithium as an anode material has been suggested. This is because it yields a cell having a very high energy density. That is, the cell that can store a substantial amount of electrical energy for a given size. Furthermore, manganese dioxide (MnO.sub.2) and Li derivatives of manganese dioxide have been shown to be good cathode materials for such lithium-based cells as these materials provide a high electrochemical potential against lithium. Moreover, MnO.sub.2 is inexpensive, environmentally friendly and readily available. As a result, considerable effort has been devoted to development of secondary Li/MnO.sub.2 cells.
Although some lithium-based cells have also met the requirement of being capable of being cycled numerous times, they unfortunately have a number of problems in practical implementation. This is because these cells are not safe under abusive conditions such as overcharging, short circuiting or exposure to high temperature.
The basic problem with this type of secondary cell is the high reactivity of the lithium deposits, which are formed on the anode during cycling, with the electrolyte. Abusive operating conditions can increase the temperature and pressure within the cell.
This is a very hazardous condition and can lead to the splitting open of the cell, an event known as venting. This venting can range from venting accompanied by a mild flame; through venting which is accompanied by vigorous flames; and to venting in which there is a violent explosion. All these venting conditions pose a considerable safety risk.
As one attempt to solve the problem of the reaction between the anode and the electrolyte, an electrolyte containing propylene carbonate (PC) and ethylene carbonate (EC), with ethers, has been proposed. This PC/EC based electrolyte has, however, not provided a safe cell under abusive operating conditions.
A different attempt at solving the problem has been to provide a porous separator having a low melting point. The intention of this is that as the temperature inside the cell rises above the melting point of the material making up the separator, it will melt and block the pores. This would prevent the flow of ions between the cathode and anode and effectively terminate the current and consequently the raise in temperature. Unfortunately though, the melting points of these separators are too high and they are not effective in preventing the reaction of the lithium with the electrolyte.
From the above then, it is apparent that none of the proposals cited above is satisfactory and the need still exists for a high energy density lithium based secondary cell which can be safely cycled many times and which is also appropriately safe under the abusive operating conditions of high temperature, overcharging and short circuiting.